[HN Gopher] Humanly Traversable Wormholes
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Humanly Traversable Wormholes
Author : nabla9
Score : 72 points
Date : 2022-03-26 15:25 UTC (7 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| dark-star wrote:
| And, as always with these kinds of papers, everything only works
| out in ADS space, which has nothing in common with how our own
| universe works...
| disentanglement wrote:
| The wormhole in this paper is actually in flat space. The
| geometry only approximates to an AdS geometry times a sphere
| close to the event horizons.
| sdoering wrote:
| ADS space? I have not yet heard that acronym.
|
| If anyone else never heard of the "Anti-de Sitter space" [0]
| here a short description from Wikipedia:
|
| > In mathematics and physics, n-dimensional anti-de Sitter
| space (AdSn) is a maximally symmetric Lorentzian manifold with
| constant negative scalar curvature. Anti-de Sitter space and de
| Sitter space are named after Willem de Sitter (1872-1934),
| professor of astronomy at Leiden University and director of the
| Leiden Observatory. Willem de Sitter and Albert Einstein worked
| together closely in Leiden in the 1920s on the spacetime
| structure of the universe.
|
| Sadly this is so far outside of my level of understanding that
| I still don't have a clue.
|
| Would love an ELI5.
|
| [0]: https://en.wikipedia.org/wiki/Anti-
| de_Sitter_space?wprov=sfl...
| zachf wrote:
| Think about 2D surfaces. Which ones are the most symmetric? A
| flat plane is a very nice space: every point is as good as
| any other (there's nothing intrinsic to any point to
| distinguish any point from any other, except arbitrarily),
| and no direction is particularly special either. A space like
| that has a lot of symmetries. A sphere also has a lot of
| symmetries, it also has no directions or points which are
| distinguished until you declare, "this is my North Pole"
| arbitrarily. (The earth isn't a perfect sphere of course and
| we can use the imperfections as the way we define north and
| south.) The last type of symmetric space looks like a saddle
| (like on a horse). It bends one way in one direction and
| bends the other way in the other direction. An idealized
| saddle also has no distinguished directions or points.
|
| The analogs of these things in higher dimensions, and where
| one of the directions is time, are important in general
| relativity. The analog of the plane is called "flat space" or
| "Minkowski space". The analog of the sphere is "de Sitter
| space". Finally, the analog of the saddle is "anti-de Sitter
| space" (usually abbreviated AdS, with a lowercase d). It's a
| bit of an odd space in a lot of ways. When you look at what
| space looks like at any given time, it's a bit like M.C.
| Escher's "Angels and Devils".
|
| Surprisingly, Anti-de Sitter space is the easiest space to
| understand quantum aspects of gravity in. That's because
| anti-de Sitter space is curved in such a way that the
| complicated stuff can be neatly separated from the easy
| stuff. You can start from something you understand well and
| turn on the complexity piece by piece. Roughly speaking it's
| because the gravitational stuff becomes less important as you
| go farther and farther away from any matter you're
| considering, in a way which is even faster than this happens
| in flat space or de Sitter space. It turns out that we can
| exactly understand everything in this gravitational theory by
| mapping the physics one-to-one to a nongravitational model
| which we understand really well. There's a lot of evidence
| that the map works perfectly. This is called the AdS/CFT
| correspondence. A lot of work goes into testing the
| correspondence and attempting to prove it, and this is a big
| research area.
|
| de Sitter space doesn't have the same desirable properties.
| Nevertheless there has been great progress in understanding
| quantum properties of de Sitter in the last year [0]. These
| results would not have been possible without understanding
| AdS first.
|
| Flat space quantum gravity remains challenging, although
| again some progress has been made recently too [1].
|
| [0] arxiv:2110.14670
|
| [1] arxiv:1905.09809 and many others
| verve_rat wrote:
| Thanks for the fantastic explanation.
|
| So there is flat space, de Sitter space, and Anti-de Sitter
| space, do we know which one most closely resembles the
| world we observe?
| zachf wrote:
| Strictly speaking it's none of them, because those are
| idealized perfectly symmetrical spaces with no matter in
| them, only dark energy, and our universe (happily) has
| matter in it :). But it's very, very close to flat,
| except not quite perfectly flat, and the best
| observational evidence leads us to believe that if you
| neglect the matter and think only about the dark energy
| part, we'd actually be living in a de Sitter spacetime.
|
| The quantity that measures this is called the
| cosmological constant. It's zero in flat space, positive
| in de Sitter and negative in anti-de Sitter. It turns out
| from measurements that our cosmological constant is
| positive but outrageously small, tiny compared to
| anything else we know about in physics. This is puzzling
| because we would love to relate it to something we
| understand already but it's hard to arrive at a result so
| small working with quantities that are considerably
| larger. So there's an interesting open question about why
| it is what it is.
| ramadis wrote:
| Just to add to the reply, the AdS/CFT correspondence (aka
| Maldacena duality) was proposed by Juan Maldacena, one of
| the authors of this paper (Humanly traversable wormholes).
| netfl0 wrote:
| Solution for traffic.
| 29athrowaway wrote:
| Maybe if you enter a wormhole, this happens:
| https://www.youtube.com/watch?v=doyaw8ipQpk
| TedDoesntTalk wrote:
| I can't believe Spore is 10 years old.
| messe wrote:
| More than that. It's 14 years old.
| shireboy wrote:
| I wasn't aware of the constraint that travel within wormhole must
| take longer than travel between the mouths. Given that, is there
| any real practical use for them? In scifi they are usually
| presented as shortcuts, but this constraint makes that seem less
| likely to ever be true.
| xwdv wrote:
| Well I think you might still experience time as a short trip
| but everything else would age as if you had completed the
| physical distance?
| remcob wrote:
| This holds true for regular travel nearing light speed too:
| For an external observer the spaceship never goes faster than
| light, but the passengers clocks slow down and they can
| experience arbitrarily high faster-than-light speeds.
|
| For some reason I never see this discussed when people talk
| about FTL travel, maybe I'm wrong?
| pcl wrote:
| Orson Scott Card's sequels to _Ender's Game_ take this into
| account.
| hiptobecubic wrote:
| I think because we use words like "faster" to mean "more
| stuff in less time," not just "feels like less time to you,
| but isn't."
| bno1 wrote:
| It's because distances and time durations contract the
| faster you travel. The passangers don't experience faster
| than light speeds, just shorter distances.
|
| This is how the twin paradox gets solved. The twin that
| leaves earth sees the trip as if it was shorter in both
| directions, so from their perspective it makes sense that
| they aged less than the twin that remained on earth.
|
| Also, if you had infinite energy and you could travel at
| the speed of light you wouldn't feel any movement or time
| passing during your trip, it would feel like instantly
| teleporting from one place to another. Photons wouldn't
| feel their existence if they could. From their perspective
| they are produced in one place and instantly absorbed in
| another place.
| imglorp wrote:
| The most practical constraint is getting to one.
|
| The paper indicates they will "resemble intermediate mass
| charged black holes". The nearest black hole at the moment,
| maybe a worm hole candidate, is V723 Monocerotis at 1500 LY
| away. This would be tens of thousands of years of travel.
|
| So perhaps we can learn from afar but not visit.
| GordonS wrote:
| I was just about to say that same thing - any time I've seen
| wormholes in sci-fi (I'm reading Peter F Hamilton right now!),
| you go through in an instant (for all observers).
|
| It's like sci-fi uses wormholes as a practical alternative to
| travelling at sub-C, relativistic speeds.
| dylan604 wrote:
| In Contact, <spoilerAlert> the pod drops instantly through
| from everyone's perspective outside the wormhole, but inside
| the wormhole 99 hours elapsed (potentially).
| tokai wrote:
| In the game Free Space there is travel time through wormholes
| (subspace jump nodes). In the finale the player intercepts
| and destroy the enemy flag ship as it is travelling through a
| wormhole to Earth.
|
| Only scifi example I can think of wormholes with travel time.
| Aerroon wrote:
| Stargate has had time travel through wormholes in a few
| cases.
| JaimeThompson wrote:
| The wormhole in Star Trek Deep Space 9 had a travel time.
| Rhinobird wrote:
| Yeah. Think teleporter instead of warp drive
| GordonS wrote:
| Yes, exactly what I meant, but said more succinctly!
| beecafe wrote:
| This constraint is needed because having a shortcut would allow
| time travel, like all FTL does.
| arcastroe wrote:
| > FTL would imply time travel
|
| But not "time travel" in the sense that you could go back and
| kill your grandfather. Only in the sense that different
| observers could not agree on the order of events.
|
| And this doesn't seem so problematic to me. There always
| seems to exist some "true" order of events that results in
| the observations experienced by all observers, even if they
| don't agree based on their own individual knowledge.
| dghf wrote:
| ETA: my comment was more than a little confused. Original
| stands below.
|
| What I _should_ have said is that different observers can
| already, in the absence of FTL, disagree about the order of
| events with space-like separation, but it doesn 't matter
| because by definition anything that happens at one such
| event can't affect anything happening at another.
|
| FTL removes that restriction -- if something can travel
| faster than light, it can be present at two space-like
| separated events -- and that's what threatens causality,
| assuming something like the Novikov consistency conjecture
| doesn't hold.
|
| ========
|
| Original comment:
|
| > Only in the sense that different observers could not
| agree on the order of events.
|
| But that's already true of events with space-like
| separation anyway. You don't need FTL travel for that.
|
| With FTL, you run the risk of observers disagreeing about
| the order of events with time-like separation, so you are
| getting into grandfather-paradox territory.
| Andrew_nenakhov wrote:
| I still don't see why this would be a problem. Substitute
| light with sound and sight with hearing. We can exceed
| the speed of sound and produce noises that some observer
| would perceive in the incorrect order (in fact, happens
| all the time with supersonic jets). And the world doesn't
| seem to break because of that, no?
| codethief wrote:
| But that's not a problem per se. The speed of light is only a
| limit _locally_ but globally there is no such thing and in
| General Relativity there is nothing preventing time travel
| (closed timelike curves) at a global level, even though the
| existence of such curves is rather unlikely.
|
| The paper notes:
|
| > Interestingly, they are allowed in the quantum theory, but
| with one catch, the time it takes to go through the wormhole
| should be longer than the time it takes to travel between the
| two mouths on the outside.
|
| Does anyone know why exactly "quantum theory" would impose
| such requirements? A priori to me it sounds like quite a
| stretch to take a local theory like quantum mechanics to make
| claims about the global topology of the universe, given that
| QM and GR haven't been unified yet. Unless of course by
| "quantum theory" Maldacena actually means "string theory" or
| "AdS/CFT" - which wouldn't surprise me at all.
| joe_the_user wrote:
| One thought I've had is that all time travel contradictions
| are based on conscious agents. Without a conscious agent
| who aims to change things, what happened is happened.
| karpierz wrote:
| That's not true, here's an example where it's billiard
| balls:
|
| https://en.wikipedia.org/wiki/Novikov_self-
| consistency_princ...
| ben_w wrote:
| IIRC (I'm not a physicist), if you combine a closed
| timelike curve with quantum mechanics, then a random
| fluctuation in the photon field (which happens all the
| time) appearing in the middle of it will go all the way
| around, meet itself, and now there are two of them, then
| four, then eight, ...
|
| But as this is in a closed loop of time, from the outside
| it goes to infinity instantly.
| zozbot234 wrote:
| A closed timelike curve simply has to be consistent. It
| might reach a state where the fluctuations simply
| interfere with one another and reach some kind of non-
| infinity fixpoint, which always exists if there are no
| discontinuities.
| ben_w wrote:
| Given the way light works, I think the expectation is
| that all of the possible wavelengths will start doing
| this all the time -- all the ones that interfere _de_
| structively will do so, all the ones that interfere _con_
| structively will also do so, but the former has a minimum
| of zero and the latter is unbounded, so one of these will
| just run off to infinity.
|
| (It does feel like trying and failing to make one,
| getting asymptotically close should release infinite free
| energy as virtual photons _almost_ get into a CTC, but
| IANAP).
| Beldin wrote:
| Huh. So maybe gamma ray bursts are wormholes
| opening/closing?
|
| Yeah, that probably only makes sense in armchair physics.
| Which is all I'm licensed for anyway, so fine with me!
| disentanglement wrote:
| > Unless of course by "quantum theory" Maldacena actually
| means "string theory" or "AdS/CFT" - which wouldn't
| surprise me at all.
|
| The wormhole solutions from the paper are semi-classical:
| they are obtained by taking the expectations value of the
| energy configuration of a quantum theory and feeding that
| into the classical Einstein equations. Therefore, no string
| theory or AdS/CFT is needed for the construction.
| [deleted]
| teeray wrote:
| > is there any real practical use for them?
|
| An infinite source of power. Place one wormhole at the top of a
| hill, another at the bottom. Send rolling generators through
| them, stop them once in awhile to swap dead batteries for
| charged ones.
| ben_w wrote:
| I suspect that (if wormholes can actually be made)
| gravitational potential would be smooth throughout, and that
| you'd be pulled _up_ as much while falling out of the top as
| you get pulled _down_ while approaching the bottom.
| teekert wrote:
| Something tells me the wormhole also costs energy.
| teeray wrote:
| Could you power the wormhole with its own energy output? Or
| would that mean the wormhole requires infinite energy to
| resolve the conundrum?
| krapp wrote:
| You can't resolve the conundrum. You can't have perfect
| efficiency or free energy. The laws of thermodynamics
| can't be cheated.
|
| No matter what you do, you will get less work from the
| system than the energy you put in, and eventually entropy
| will eat everything. It doesn't matter if it works on
| paper, we don't live on paper.
| JPLeRouzic wrote:
| Or the wormhole will eat the place of the proposed
| experimentation?
| nabla9 wrote:
| Published in: Phys.Rev.D 103 (2021) 6, 066007
| Published: Mar 9, 2021 DOI:10.1103/PhysRevD.103.066007
|
| >. We have not given any plausible mechanism for their formation.
| We have only argued that they are configurations allowed by the
| equations.
| DennisP wrote:
| Also only "in some previously considered theories for physics
| beyond the Standard Model," and
|
| > we engage in some "science fiction". Namely, we will
| introduce a dark sector with desirable properties for
| constructing macroscopic traversable wormholes.
|
| Seems unsurprising that if you purposely introduce hypothetical
| physics with desirable properties for making traversable
| wormholes, then it turns out the physics supports traversable
| wormholes.
| at_a_remove wrote:
| I have typically framed this as "Once you allow for one
| impossible thing, other impossible things follow."
| emteycz wrote:
| Sure. But it still seems interesting to see in what ways you
| must bend the rules to make it possible. Consider that the
| goal here is not really to produce working human-traversable
| wormholes, but rather to learn more about various models of
| physics and how they behave in extreme conditions.
| Ansil849 wrote:
| > Consider that the goal here is not really to produce
| working human-traversable wormholes
|
| Then the paper should not have the clickbaity title
| "Humanly traversable wormholes".
| Rusky wrote:
| I dunno, this feels like a "no fun allowed" sort of rule.
| The target audience clearly has some background in this
| area to tell the difference, at which point this is just
| a fun joke-y title.
| zachf wrote:
| I can see why it might look like that from the outside, but
| the set of mathematical results that led up to it actually
| come from the angle of, "let's try to prove that traversable
| wormholes and the negative energy densities required to
| create them are impossible." In trying to prove that, you
| learn a lot of interesting things along the way:
|
| 1. Negative energy densities are a universal prediction of
| all quantum field theories, and therefore are not as
| outrageous to think about as one might naively believe. [0]
|
| 2. The amount of negative energy density permitted by quantum
| field theory is NOT enough to support traversable wormholes,
| as long as you make some mild assumptions about the behavior
| of spacetime. [1]
|
| 3. Those mild assumptions seem NOT to be required by string
| theory, and string theory supports some solutions with
| traversable wormholes. [2]
|
| 4. Those solutions of string theory appear to be self
| consistent in an unusual and novel way, which is why string
| theorists like Juan Maldacena find them interesting. [3]
|
| Every step is all super nontrivial and tells us something new
| about the mathematics of spacetime. Of course they're models,
| maybe they all rest on some faulty assumption about nature
| that will later turn out to be wrong. So the final pillar of
| the story, the one that's hard to communicate without
| spending years of your life studying it, is that every
| remaining assumptions about quantum gravity that goes into
| these arguments appear to be impossible to get rid of without
| having horrible consequences where spacetime can't become
| smooth or relativistic at large scales. So these models are
| at least very plausible even though the ultimate truth will
| have to come from experiments.
|
| [0] Proof appears many places, I like the one in
| arxiv:1803.04993 on page 11
|
| [1] arxiv:1010.5513
|
| [2] arxiv:1608.05687
|
| [3] the paper linked by OP :)
| DennisP wrote:
| Thanks, that does sound more plausible than I expected.
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